The present invention relates to microdischarge devices and, in particular, to novel structures for light emitting devices and low-cost methods of producing ultraviolet or visible light.
It has long been known that electrical discharges are efficient sources of light and, today, gas discharge lamps (including fluorescent sources, and metal-halide, sodium, or mercury arc lamps) account for most of the world""s light-generating capacity (several billion watts on a continuous basis). Most of these devices are, unfortunately, bulky and frequently have fragile quartz or glass envelopes and require expensive mounting fixtures. In addition to general lighting, discharges produce ultraviolet and visible light for other purposes such as germicidal applications (disinfecting surfaces and tissue), cleaning electronic and optical surfaces in manufacturing, and activating light-sensitive molecules for medical treatments and diagnostics.
Although microdischarges were demonstrated by A. D. White in 1959, only recently were microdischarge devices fabricated in silicon by techniques developed in the integrated-circuit industry. As shown in FIG. 1, a microdischarge device 100 fabricated in silicon had a cylindrical channel (microcavity) 102 in the cathode 104 of the device 100. The semiconductor cathode 104 was affixed to a copper heat sink with conductive epoxy. The anode 106 for the microdischarge device 100 was typically a metal film such as Ni/Cr. A thin dielectric layer 108 deposited onto the silicon electrically insulates the cathode 104 from the anode 106. When the channel 102 is filled with the desired gas and the appropriate voltage imposed between the cathode 104 from the anode 106, a discharge is ignited in the channel 102.
Microdischarges have several distinct advantages over conventional discharges. Since the diameter of single cylindrical microdischarge devices, for example, is typically less than 400-500 xcexcm, each device offers the spatial resolution that is desirable for a pixel in a display. Also, the small physical dimensions of microdischarges allows them to operate at pressures much higher than those accessible to conventional, macroscopic discharges. When the diameter of a cylindrical microdischarge device is, for example, on the order of 200-300 xcexcm or less, the device will operate at pressures as high as atmospheric pressure and beyond. In contrast, standard fluorescent lamps, for example, operate at pressures typically less than 1% of atmospheric pressure.
Despite their applications in several areas, including optoelectronics and sensors, microdischarge devices have several drawbacks. For example, extracting optical power from deep cylindrical cavities is frequently inefficient. If the cylindrical cathode for a microdischarge is too deep, it will be difficult for photons produced below the surface of the cathode to escape. Furthermore, the conventional microdischarge devices require an insulating/dielectric layer fabricated from a material different from that of than either the anode or cathode. The presence of this layer complicates fabrication of the device. For example, SiO2 films, polymers, glass, quartz and mica have been used as the insulating layer. However, in such a three-layer microdischarge device, drilling the top layer with a laser is straightforward, but ablating the SiO2 layer is not, and results in a cylindrical channel that is often not clean. The device quality thus is deteriorated.
In view of the above, novel microdischarge devices and fabrication methods are provided.
In one embodiment of the invention, the discharge device comprises a diode with a channel that extends through the surface of at least one of the first and second layers of the diode. A gas is disposed within the channel.
The diode may be a p-n diode, p-i-n diode, or Schottky diode. In addition, a dielectric layer and an electrode may be formed on the diode and may be biased independently of the diode. The dielectric layer may be formed from a plurality of films with at least one of the films having a dielectric constant different from at least one other of the films. A conducting screen may be disposed on at least one end of the channel. Similarly, an optically transmissive sealing material, which does not substantially absorb light of a wavelength emitted by the gas when the gas is electrically excited, may be used to seal the channel. An optically transmissive protective surface to protect the surface of the sealing material may be disposed between the sealing material and the surface of the diode.
An annular chamfer that widens the channel may be introduced to permit coupling of the discharge to an optical fiber. A bias resistor may be connected in series with the diode to regulate light output of the device.
A plurality of devices may be arranged in an array. The array may be divided into independently excited sub-arrays. The sub-arrays may have at most one of the voltages, applied to the first and second layers, in common.
In another embodiment of the invention, a method of fabricating a discharge device comprises forming a channel extending from a surface of the diode at least through a depletion region of the diode and introducing a gas to the channel.
The method may further comprise exciting the gas to form a discharge by reverse biasing the diode. In addition, the method may further comprise selecting the gas, determining an optimum reverse breakdown voltage for excitation of the gas, and selecting material of the diode prior to forming the channel.
Further, the method may comprise extending the depletion region through an intermediate semiconductor layer of the diode having a lower electrical conductivity than layers that establish the depletion region or biasing a dielectric layer and an electrode layer formed on the surface of the diode independently from the diode. The method may additionally comprise forming an annular chamfer that widens the channel and further coupling an optical fiber to the annular chamfer.
The method may comprise altering the electric field present in the channel by affixing a conducting screen to at least one end of the channel and additionally coating the screen with a phosphor or electroluminescent material. The method may also comprise sealing the channel with an optically transmissive sealing material that does not substantially absorb light of a wavelength emitted by the gas when the gas discharges.
The method may comprise arranging a plurality of the devices in an array and further dividing the array into independently excited sub-arrays.
The following figures and detailed description of the preferred embodiments will more clearly demonstrate these and other advantages of the invention.